A conventional incandescent lamp used at home, in an office or a factory has large power consumption, a large degree of heat dissipation, and a short life-span. A fluorescent lamp that supplements disadvantages of the incandescent lamp has low power consumption compared to the incandescent lamp but still has a short life-span. Thus, in these days, a light emitting diode (LED) illumination device that uses an LED having low power consumption and a dramatically-increasing life-span compared to the incandescent lamp has been widely used.
In addition, since brightness at an LED load is uniform only when a constant current flows through the LED load, it is essential that the constant current flows through the LED load in the LED driving circuit.
However, a switching mode power supply (SMPS) method illustrated in FIG. 1 is mainly used in the related art. The SMPS method is a method, whereby an alternating current (AC) power supply 10 is rectified by a rectifier circuit 11, such as a bridge circuit, and then is smoothed by a capacitor C after passing through an SMPS 12 and is supplied to an LED load 14 and a constant load current corresponding to desired brightness flows by adjusting a duty ratio at which a load current is detected by a voltage applied to a resistor Rs, the voltage applied to the resistor Rs is applied to a pulse width modulation (PWM) driving unit 13 and a switch SW is turned on/off.
However, since the LED driving circuit using the SMPS method requires the SMPS 12 having a complicated configuration and the PWM driving unit 13, in a small-sized LED illumination apparatus, such as an LED lamp, it is difficult to install components for these circuits in a narrow space of the LED lamp. In particular, these circuits operate using a pulse method. Thus, many harmonic waves are generated, and an apparatus for suppressing emission of electromagnetic waves resulting from the harmonic waves is additionally required. Of course, the LED driving circuit using the SMPS method of FIG. 1 can boost a voltage using an inductor L and thus can drive more LEDs. An input current of the LED driving circuit follows an input voltage using a sawtooth method, and a duration in which the input current flows, is increased, and a power factor is improved.
Meanwhile, in another related art considering the problems of the LED driving apparatus using the SMPS method, an LED driving circuit using a constant-current circuit method, whereby a constant current flows through an LED load 22 by installing a constant-current circuit 23 using an analog circuit method, is used, as illustrated in FIG. 2. Since the constant-current circuit 23 using this method uses a simple analog circuit shown in FIG. 3 (in FIG. 3, a transistor is used but an operational (OP) amplifier can also be used), a circuit configuration is simple, and the constant-current circuit 23 using the analog circuit method is used so that there are no worries about generation of electromagnetic waves resulting from harmonic waves.
However, the LED driving circuit using the constant-current circuit method of FIG. 2 is not suitable for a general purpose illumination apparatus due to flicker that occurs in the LED driving circuit.
In order to solve the problem of flicker of the LED driving circuit of FIG. 2, the LED driving circuit using the constant-current circuit method according to another related art removes flicker by adding a capacitor C, as illustrated in FIG. 4.
However, for example, in the LED driving circuit using the constant-current circuit method of FIG. 2, if an input AC voltage 20 (Vin) is 220 Vrms (a peak value is about 310 V) and a load voltage (output voltage) VL applied to the LED load 22 when the LED load 22 is turned on, is 235 V, the input voltage Vin and an input current Iin have waveforms of FIG. 5. Here, an output current IL is equal to the input current Iin, because a circuit is a single loop.
Thus, if a voltage applied to a constant-current source 23 when the LED load 22 starts being turned on, is 5 V, the LED load 22 is turned on in a section in which the input voltage Vin is equal to or greater than 240 V (235 V+5 V), and in this case, the load current IL that is a constant current set in the constant-current source 23 flows through the LED load 22.
However, in general, efficiency is defined as output power PL at an output terminal=[output current IL×output voltage VL] with respect to input power Pin at an input terminal=[input current Iin×input voltage Vin]. Since the driving circuit of FIG. 2 is a single loop, the output current IL and the input current Iin that flow through the LED load 22 are constant currents, and input current Iin=output current IL.
In this case, the output voltage VL applied to the LED load 22 is constant as 235 V but the waveform of the input voltage Vin draws the curve of FIG. 5. Thus, if efficiency is calculated using an approximate method in consideration of FIG. 5, the efficiency is about 86%, like in the following Equation. That is, only 86% of supplied power is used to emit light from the LED load 22, and a power loss of about 14% occurs.Efficiency=[(IL)×(235+235+235+235)/[(Iin)×(240+260+280+310)]=940/1090=86.2%
Although the efficiency of about 86% is high in the field of a general illumination apparatus, an LED driving apparatus having higher efficiency is required in an LED illumination apparatus that pursues power-saving.
However, it is generally very difficult to make efficiency 90% or more by further improving comparatively high efficiency of 86%.